netdev - Re: [RGC PATCH v3.10] net: sched: validate that class is found in qdisc_tree_decrease

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Date:	Mon, 20 Jul 2015 11:45:12 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Alex Gartrell <alexgartrell@...il.com>
Cc:	Alex Gartrell <agartrell@...com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	kernel-team <kernel-team@...com>
Subject: Re: [RGC PATCH v3.10] net: sched: validate that class is found in qdisc_tree_decrease_qlen

On Fri, Jul 17, 2015 at 5:27 PM, Alex Gartrell <alexgartrell@...il.com> wrote:
> On Fri, Jul 17, 2015 at 5:10 PM, Cong Wang <cwang@...pensource.com> wrote:
>> Hmm, but in htb_delete() we do reset the leaf qdisc before removing the
>> class from ha
>>
>>         if (!cl->level) {
>>                 qlen = cl->un.leaf.q->q.qlen;
>>                 qdisc_reset(cl->un.leaf.q);
>>                 qdisc_tree_decrease_qlen(cl->un.leaf.q, qlen);
>>         }
>>
>> therefore, the leaf fq_codel qdisc is supposed to have 0 skb after that,
>> that is, the second fq_codel_reset() is supposed to return NULL immediately?
>> Why is qdidsc_tree_decrease_qlen() called in the second fq_codel_reset()?
>>
>> Or there is a race condition between ->delete() and ->put()? In which new
>> skb can be enqueued?
>
> This makes the most sense to me, but I have ~32 hours of experience
> with this subsystem :)
>
> Taking that for granted, it seems like it'd be appropriate to note the
> invariant in the code i've changed with a WARN_ON and to skip it, and
> then to otherwise find a way to close the hole.  Do you agree?
>

Your patch _does_ make sense for me, I was just trying to check
if there is any better way to fix it. :)

I was thinking about locking the qdisc tree before ->get(), but it is not
easy and may not worth the effort either. So, unless others have a
better idea here, I think your patch is fine, just please update the
changelog and rebase it against latest -net tree.

Thanks!
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